bill - You've got the whole history of video packed into these questions, including known unknowns and unknown unknowns. This will be a very rushed answer, and therefore probably a little bit sloppy and overly long. It will gloss over a lot.
Let me start by asking you a few questions to narrow down the scope of the answer...
- country of origination?
- where will the primary audience probably live?
- what are the videos for? Eg, is definition of tiny detail important (as might be, say, with screen capture of an interface)? Or, are these general purpose videos?
- hi-def or standard-def?
Is it still necessary to format DVDs in NTSC, PAL or SECAM for them to be seen in each region?
I should note up front that these are standard definition
video formats for various regions. Playable DVDs are standard-def, which in DVDs are encoded/transcoded to MPEG-2, so they do draw from the NTSC, PAL and SECAM standards.
They are not the same as Blu-ray and newer home digital video standards, natively encoded in H.264/MPEG-4, which, as I understand it, leave these formats behind.
Quick overview answer with regard to DVDs is most everything depends on what you're playing back on... but, in general, "playable DVDs" need to be NTSC, PAL, or SECAM specific when you encode/author them... and that stand-alone DVD players in PAL and SECAM regions are more likely to be able to play NTSC than the other way around.
"All-region players" have a converter chip built into them. Converting among highly compressed sources can occasionally produce odd results. I own an all-region player. Most Americans (NTSC) don't. (The Japanese variant of NTSC, which might concern you, is only very slightly different from the American version... can be adjusted with the brightness control.)
I should add, btw, that if you're producing these disks, you probably won't limit by region... that was thrown in by the studios and distribution companies... but the technical issues will still exist.
All DVD players, though, even apart from the different video standards, are different. The DVD "standard" has never been fully implemented on any one player. This usually effects menu options. Can also affect data rate they can handle.
It's easier to play all regions on a computer, and in general, because of their greater computing power, computers will handle a greater range of DVDs than stand-alone players. Duplicated disks won't always play on all players, but I think you get about 80% of them.
With digital playback, data refresh rates on monitors used to enter into it. I'm not sure how much of a factor that still is. Frame rates are 25fps for PAL, and c30 fps for NTSC. Laptops may be most immune to this issue.
I assume you're talking about duplicated disks (burned copies) rather than replicated (mass produced from glass stampers). I've been advised to stay with DVD-R disks, and Taiyo Yuden is the best brand. After the earthquake, when Taiyo Yuden disks were hard to get, I went to Falcon brand disks (Tayo Yuden engineers, company in United Arab Emirites).
Web issues are complicated by evolving standards, but they are all attempting to be compatible to a degree with the newer digital video standards.
Into the near future, I'm not sure whether web video standards for HTML5 have been fully nailed down. H.264/MPEG4 video carries a patent burden, whereas some other formats are under consideration because they do not. However, H.264 is widely used in Blu-ray, etc, as well as in some video cameras.
Getting back to MPEG-2 / MPEG-4... converting MPEG-2 to MPEG-4 is not likely to give good results, particular for fine detail. In general, if you do convert between formats, do your conversion on the least compressed sources you can find. Etc. I know I've left a bunch out. ;)